pH-dependent rearrangement determines the iron-activation and antitumor activity of artemisinins.

The action mechanisms of artemisinins remains elusive for decades, and one long-standing question is whether the indispensable peroxide group is activated by iron or heme. Although heme usually reacts faster with artemisinins than iron does, we have found that rearrangement of dihydroartemisinin (DHA) into monoketo-aldehyde-peroxyhemiacetal (MKA) under physiological conditions can significantly enhance its reaction towards iron. The rearrangement is pH-dependent and the derived MKA is identified by LC-MS in the cellular metabolites of DHA in cancer cells. MKA reacts quickly with ferrous irons to afford reactive carbon-centered radicals and can inhibit enzyme activities in vitro. Moreover, MKA oxidizes ferrous irons to ferric irons, which may explain the effect of DHA on decreasing cellular labile iron pool (LIP). Both addition of exogenous iron and increase in LIP via triggering ferroptosis can enhance the cytotoxicity of DHA against cancer cells. While artesunate (ATS) can also decompose to MKA after hydrolyzing into DHA, the other artemisinins of lower antitumor activity, e.g. artemisinin (ART), artemether (ATM) and arteether (ATE), exhibit negligible hydrolysis and rearrangement under the same conditions. Our study reveals the vital role of molecular rearrangement to the activation and activity of artemisinins and provides a new strategy for designing antitumor molecules containing endoperoxide group.